Question

Why only Boran or cadmium rods are used to control a nuclear reaction?

Answer 1

A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium. Chemical elements with a sufficiently high capture cross section for neutrons include silver, indium and cadmium. Other elements that can be used include boron, cobalt, hafnium, gadolinium, and europium. Because these elements have different capture cross sections for neutrons of varying energies the compositions of the control rods must be designed for the neutron spectrum of the reactor it is supposed to control.

For details go through below given description.

In nuclear reactors nuclear energy is generated under controlled fission process of uranium. The process involves breaking up of heavier U atom into two lighter elements, release of 2–3 neutrons and energy. Excess neutrons produced in the process will hit another atoms of U and the process, which is a chain reaction thus goes on continuing within the core of the reactor. If all the neutrons produced in the process are used in fission process an enormous amount of energy and pressure are build up with in the reactor in short span of time which will destabilize the reactor system and ultimately the reactor core will burst which will cause devastating the environment by release of gaseous fission products info the atmosphere. So in order to control the rate of chain reaction some mechanism is required. I.e to control the numbers or availability of neutrons optimum in the process I.e 1:1 ratio atom: neutrons the excess neutrons are to be removed from the reactor core. It can not be possible to siphon out the excess neutrons from the reactor while nuclear chain reaction is going on in closed system. The only way is to remove the excess neutrons through some elements which possess strong neutrons absorption capacity. One such element is boron which has larger neutrons absorption capacity. By regulating the movement or insertion of boron rods into the reactor core, while fission process is in progress, the excess neutrons produced during the chain reaction are absorbed by boron which will regulate the availability of neutrons to sustain the nuclear chain reaction and constant accumulation of heat in the reactor core, which is subsequently removed to the turbain circuit . If the reactor needs any maintenance to shout down the reactor, boron rods also called as controlled rods in nuclear industry are inserted to stop the nuclear chain reaction.